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Thermal resistance evaluation

In order to evaluate the thermal resistance of a fabric or a multilayered fabric system under steady-state condition, the temperature of the test plate (shown in Fig. 5.23) is kept at 35 ±0.5°C (without fluctuating more than ±0.1 °C during testing) to simulate human body temperature. The ambient air of the plate can be set at 04-25°C temperature (without fluctuating more than ±0.1°C during testing) and 20-80% relative humidity (without fluctuating more than ±4% during testing); and the air flow over the plate can be controlled at a speed of 0.5-1 m/s (without fluctuating more than ±0.1 m/s during testing). The air temperature is selected such that it will generate a power level in the middle range of the instrument while maintaining a plate temperature of 35°C, and thicker fabrics always need to be tested at lower air temperatures; any relative humidity can be selected (within the range of 20-80%) as it has the least impact on the thermal resistance of a fabric or a multilayered fabric system under steady-state conditions. After setting the experimental parameters (the temperature of the plate and ambient air, relative humidity of the ambient air, and the ambient air velocity) as per requirements, a fabric or a multilayered fabric system specimen is placed on the test plate with the side normally facing the human body towards the plate; in the case of multiple layers, it is necessary to arrange the specimens on the plate as on the human body. When the specimen reaches the steady-state condition, the temperature at the plate surface and the ambient air temperature on the specimen’s surface are determined. Using the temperature values of the plate surface (Ts) and the air temperature (Ta), the total thermal resistance (Rct) provided by the specimen and the plate boundary air layer is calculated using Eq. (5.19) given in the ASTM F 1868 standard, where, Rct = total thermal resistance provided by the specimen and air layer (Km2/W); A = area of the plate (m2); Ts = surface temperature of the plate (°C); Ta = air temperature (°C); and Hc = power input (W).

Here, it seems that a significant amount of trapped air on the boundary of the bare hot plate surface contributes to the Rct. Thus, the intrinsic thermal resistance (Rcf) of the specimen can be determined by subtracting the thermal resistance (Rcpb) of the bare plate from the Rct (Eq. 5.20). This Rcf determination process was developed based on the assumption that the air layer resistance measured on the bare plate is the same as the air layer resistance on the surface of the tested specimen. Although this Rcf determination process is easy, the assumption is not always true. This is because the heat flux from the bare plate is often greater than the heat flux from the surface of the tested fabric specimen, particularly for thick specimens, unless the temperature difference between the plate surface and the air is adjusted to compensate for the added thermal resistance of the tested specimen. Additionally, the emissivity of the plate may not be comparable to the emissivity of the tested specimen, and this may affect the radiant heat flux through the air layer [385]. One point of note: the Rct/Rcf value obtained from the Eq. (5.19) or (5.20) is an SI-unit; these values needs to be multiplied by 6.45 to convert the SI-unit to a more commonly used thermal resistance unit “clo” [386]. Here, 1 “clo” is equivalent to 0.155 K m2/W, and the value of the “clo” was selected as roughly the thermal resistance (insulation) value of typical indoor clothing that can keep a resting man (producing heat at the rate of 58 W/m2) comfortable in an environment at 21°C with air movement at 0.1 m/s.

Although the ASTM F1868 standard is widely used to evaluate the thermal resistance, this standard possesses several limitations [377,385,386]. For example, the tested specimen should be large enough to cover the surface of the test plate and the guard section completely in order to prevent any heat loss; a specimen thicker than 0.5 cm should be tested on a plate with a large guard section to prevent any heat loss through the fabrics’ edges (if a large guard is not used, a lower thermal resistance value will be measured); the tested specimen should be free from any undesirable wrinkles or bubbles, and there should be no unwanted air gap between the plate and the specimen, as well as within the specimen, to accurately evaluate the thermal resistance value. The ASTM F 1868 standard method is unsuitable to accurately evaluate the thermal resistances of very thin fabric specimens because they do not adhere to the plate well. Air tends to become trapped under the specimens, which erroneously enhances the evaluated thermal resistance values, while the standard is limited to evaluating thermal resistance values within a range of 0.002-0.5 K m2/W.

Similar to the ASTM F 1868 standard, the ISO 11092 standard can be used to evaluate the thermal resistance of any fabrics/films/battings using the following experimental parameters under steady-state conditions: a test plate temperature of 35°C, an ambient air temperature of 20°C, an ambient air relative humidity of 65%, and an ambient air velocity of 1 m/s (horizontal air flow with a 5-10% level of turbulence). In this ISO 11092 standard, the horizontal air flow may contribute to the nonuniformity in the ambient air temperature or humidity (or both) across the plate, particularly when the heat flux from the plate is high (ie, the test is performed on the bare plate and/or thin fabrics). This is because the hot test plate is exposed to the air stream first and the air gains heat or humidity or both as it crosses the test section. This kind of problem may not be prominent in the case of vertical air flow, and vertical air flow can realistically simulate real life conditions when clothing is worn; however, only a few labs have a hood capable of generating vertical air flow [385]. Another standard is available to evaluate thermal resistance: ASTM D 1518. This standard evaluates the thermal resistance of a batting or a batting/fabric system’s specimen, under steady-state conditions. It measures the heat transfer from a warm, dry, constant-temperature, horizontal flat-plate up through a layer of the test specimen to a cool atmosphere and calculates the thermal resistance of the specimen; the measurements are made under still air over the specimen. It seems that the ASTM D 1518 standard is mainly limited to testing fabric used in cold-weather protective clothing, sleeping bags, and bedding systems, along with several other limitations. The standard is restricted to determinations on specimens of battings and layered batting/fabric assemblies having an intrinsic thermal resistance from 0.1 to 1.5 K m2/W and thicknesses not in excess of 50 mm; it is also limited to evaluating the thermal resistance of fabric under a still air condition, which is not acceptable in all scenarios, Additionally, this standard was developed with a conception of fabric as a homogeneous material, although most fabrics have heterogeneous structures (ie, mixtures of fiber and air, composite) [385,388].

Despite differing parameters, all the above-mentioned standards (ASTM F 1868, ISO 11092, ASTM D 1518) evaluated thermal resistance using a single hot test plate. This single-plate method can simulate a test in which tested fabrics are exposed to ambient environment. However, this type of simulation is not always acceptable; sometimes, there is a need to test a fabric that is shielded from the ambient environment by an outer layer. For example, the thermal battings/liners in firefighters’ clothing are always shielded from the ambient environment by the shell fabrics. In order to evaluate the thermal resistance of a fabric in a shielded condition, a two-plate method is required [377]. In this test method, a fabric specimen is placed on a hot plate (representing the human body) and then a cold plate (representing the outer layer) is placed on the specimen—this method utilizes heat flow principle from the hot plate towards the cold plate through the specimen, and measurement of the temperature gradient through the tested specimen is made using thermocouples (Fig. 5.24) [377,389,390]. Presently, the ASTM C 177 standard test method is also available to evaluate the thermal resistance of any material under steady-state conditions using a wide range of experimental parameters; however, this method is not specifically designed for textile materials [385,391].

Schematic diagram of the two-plate test method to evaluate thermal resistance

Fig. 5.24 Schematic diagram of the two-plate test method to evaluate thermal resistance.

 
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